Bumpless Restart (PID)

The PID instruction can interact with the 1756 analog output modules to support a bumpless restart when the controller changes from Program to Run mode or when the controller powers up.

When a 1756 analog output module loses communications with the controller or senses that the controller is in Program mode, the analog output module sets its outputs to the fault condition values you specified when you configured the module. When the controller then returns to Run mode or re-establishes communications with the analog output module, you can have the PID instruction automatically reset its control variable output equal to the analog output by using the Inhold bit and Inhold Value parameters on the PID instruction.

Instructions for setting a bumpless restart

Do this	Details
Configure the the channel of the 1756 analog output module that receives the control variable from the PID instruction	Select the Hold for initialization box on the properties page for the specific channel of the module. This tells the analog output module that when the controller returns to Run mode or re-establishes communications with the module, the module should hold the analog output at its current value until the value sent from the controller matches (within 0.1% of span) the current value used by the output channel. The output of of the channel ramps to the currently held output value by making use of the .BIAS term. This ramping is similar to auto bumpless transfer.
Enter the Inhold bit tag and Inhold Value tag in the PID instruction	The 1756 analog output module returns two values for each channel in its input data structure. The InHold status bit (.Ch2InHold, for example), when true, indicates that the analog output channel is holding its value. The Data readback value (.Ch2Data, for example) shows the current output value in engineering units. Enter the tag of the InHold status bit as the InHold bit parameter of the PID instruction. Enter the tag of the Data readback value as the Inhold Value parameter. When he Inhold bit is true, the PID instruction moves the Inhold Value into the Control variable output and re-initializes to support a bumpless restart at that value. When the analog output module receives this value back from the controller, it turns off the InHold status bit, which allows the PID instruction to start controlling normally.

Do this

Details

Configure the the channel of the 1756 analog output module that receives the control variable from the PID instruction

Select the Hold for initialization
box on the properties page for the specific channel of the module.

This tells the analog output module that when the controller returns to Run mode or re-establishes communications with the module, the module should hold the analog output at its current value until the value sent from the controller matches (within 0.1% of span) the current value used by the output channel. The output of of the channel ramps to the currently held output value by making use of the .BIAS term. This ramping is similar to auto bumpless transfer.

Enter the Inhold bit tag and Inhold Value tag in the PID instruction

The 1756 analog output module returns two values for each channel in its input data structure. The InHold status bit (.Ch2InHold, for example), when true, indicates that the analog output channel is holding its value. The Data readback value (.Ch2Data, for example) shows the current output value in engineering units.

Enter the tag of the InHold status bit as the InHold bit parameter of the PID instruction. Enter the tag of the Data readback value as the Inhold Value parameter.

When he Inhold bit is true, the PID instruction moves the Inhold Value into the Control variable output and re-initializes to support a bumpless restart at that value. When the analog output module receives this value back from the controller, it turns off the InHold status bit, which allows the PID instruction to start controlling normally.

Special Instructions

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